The present application claims the benefit of U.S. Provisional Application No. 60/306,742 filed Jul. 21, 2001.
1. Technical Field
The present invention relates to exhaust systems for combustion engines; and more specifically, it relates to configurations in exhaust systems for marine combustion engines designed to prevent the detrimental back flow of sea water through the exhaust system into the cylinders of the combustion engine.
2. Background Information
The present invention takes the form of a design characteristic or adaptation to the exhaust system of a combustion engine utilized in a marine vehicle such as a sport-boat. Referring to typical arrangements in boats of this nature, one configuration includes a stern-drive that is connected to the powering combustion engine across a transom wall of the boat. In such a configuration, the engine itself is located in an engine compartment at the rear area of the boat. Exhaust produced by the engine is ported through the transom, and typically, at least partially passes through the stern-drive assembly to ultimately be released to the environment. As the exhaust passes from the engine to the stern-drive unit, the elevated exhaust pressure is permitted to drop substantially to atmospheric pressure. Because the ultimate porting of the exhaust is below the boat""s static waterline, at least when the boat is at standstill and moving at slow speeds, the pressure of the exhaust must remain sufficiently above atmospheric to prevent the sea water""s inundation. When the engine is running, there is ample exhaust pressure to overcome the hydrostatic pressure of the water being exerted thereupon and prevent sea water from moving into the exhaust arrangement and toward the combustion engine.
At least one typical marine engine configuration takes the form of a V8 combustion engine having eight cylinders oriented at a 90 degree V-angle. Four of the eight cylinders are arranged in banks on each of two sides of the engine; the exhaust from these two cylinder banks are respectively ported to two different exhaust manifolds. Connected to each manifold is a riser which generally provides a right-angle turn or elbow terminating in a substantially horizontally oriented exhaust pipe. These two exhaust pipes are directed rearwardly away from the engine and toward the rear transom of the boat. At a position in the mid-length of each exhaust pipe, spent cooling sea water is often introduced thereinto after having been circulated through the engine for cooling purposes.
At the rearward region of the engine, a Y-configured piping arrangement is utilized to combine the two exiting exhaust streams into one. This Y-configured piping arrangement is substantially vertically oriented with two upper V-arranged arms oriented above, and in fluid communication with, an upright leg portion which forms the bottom of the Y. Based on this configuration, exhaust passes downwardly through each arm of the V-arrangement, further down through the upright leg, and then out at a bottom portion thereof to the stern-drive unit for ultimate release. When the boat is at rest and the engine is not running, the static waterline is typically, with respect to vertical, positioned approximately midway up each of the V-arranged arms.
Based on historical experience, and now experimentation, it has been appreciated that during certain shutdown procedures, and rather unpredictably, sea water is permitted to rise up through the Y-configured piping arrangement to a level that the liquid detrimentally flows back into the cylinders of the combustion engine.
In another situation, it has also been appreciated that sea water can be permitted to flow back into the engine""s cylinders. This situation occurs when the engine is abruptly shut down when the boat is traveling at high speeds. When traveling at cruise speeds, a sport boat is typically planing across the top of the water. If the engine is shutdown under these conditions, the boat settles quickly back into a displacement orientation in the water. Still further, as the boat is settling and quickly slowing, a surge of water is experienced at the back or transom of the boat. The resulting hydraulic pressure upon the stern drive can also cause sea water to travel up the Y-configured piping arrangement resulting in flow back into the engine""s cylinder(s).
Based on one embodiment of an exemplary V8 engine, the operation of the engine is characterized as xe2x80x9codd firing on each bank.xe2x80x9d That is to say, these are not alternating cylinders firing at 180 degrees to one another, as is more typical. Instead you have a cylinder sequence that can be 90 degrees, 180 degrees, and 270 degrees apart. It is theorized that this operational sequence, under certain stroke conditions, can cause unbalanced exhaust pulses from each of the two different cylinder banks or manifolds. As a result, the exhaust pressures being experienced in the two arms of the V-arrangement are not always equal. Normally this does not present a problem, unless cessation of the engine""s operation (shutdown) is caused to occur when the engine is at one of these imbalanced sequences. When this pressure imbalance does occur and the boat begins to settle into the water after shutdown, the hydrostatic pressure of the external sea water progressively rises up the lower upright leg of the Y-arrangement. As the rising water encounters the two V-oriented arms and the out-of-balance exhaust pressures contained therebetween, the rising water is directed up the lower pressured arm, at a much greater rate than if the arms were in balance with respect to pressure. Upon reaching the upper portion of the respective V-arm, and especially the low-pressure arm, the water flows over the riser and is permitted to drain back into the engine.
It is under these irregularly occurring imbalanced exhaust pressure shutdown conditions that detrimental flow-back into the engine may occur. When such flow-back has occurred, several highly undesirable affects can be encountered upon restarting the engine. Primarily, these affects stem from what is termed hydro-lock. Hydro-lock describes the situation in which a sufficient volume of water has been introduced into the cylinder so that upon a compression stroke, where a volume of fuel and air mixture is designed to be compressed to a clearance volume, the substituted water volume which is relatively incompressible in comparison to such a fuel/air mixture causes the engine to seize-up. This is also referred to as xe2x80x9cgoing hydraulicxe2x80x9d with regard to the engine""s operation. When this occurs, any number of detrimental conditions may be caused including bent connecting rods, a broken starter and cracked engine blocks.
It is for these reasons that the present invention has been designed in order to help prevent engine damage as a result of flow back of sea water through the exhaust-pipe arrangement of a marine vehicle.
The present invention provides an arrangement which successfully prevents the described flow back of sea water through the exhaust-pipe arrangement to the combustion engine associated therewith. As indicated above, one condition which can cause the flow back of sea water into the cylinders of the engine after shutdown results because of the intermittently occurring pressure imbalance induced in the parallelly operating exhaust pipes or arms that are positioned downstream from the exhaust manifolds of the engine. The elegant solution of the present invention is to equalize this pressure imbalance between the exhaust pipe arms so that rising water, typically under hydrostatic pressure, does not rise a substantially greater distance in any one arm than the other(s). One of the reasons that the solution described herein is considered to be elegant is its simplicity in form and ease in implementation. By coupling a connective conduit between the arms, equalization is readily facilitated, provided the conduit is of sufficient size to accommodate the passage of fluid at a rate necessary to prevent sufficient rising in the lower-pressured pipe which would otherwise result in spill-over into the engine""s cylinder(s).
In at least one embodiment, the invention takes the form of an arrangement for a marine combustion engine exhaust system. The arrangement includes at least two substantially upstanding exhaust pipe arms that are coupled together in common closed fluid communication with a stem exhaust pipe. A pressure equalizer is fluidly connected between the upstanding exhaust pipe arms. The pressure equalizer is configured to provide sufficient fluid passage from each exhaust pipe arm to another to prevent flow back of sea water to a marine combustion engine associated therewith.